In 1981 Carver Mead pointed out that a computer is a thermodynamic engine that sucks entropy out of data, turns that entropy into heat, and dumps the heat into the room. Today our ability to get that waste heat out of semiconductor circuits and into the room at a reasonable cost limits the density and clock speed of those circuits.
Cooling technologies are available that can transport very high densities of waste heat For example the combined fluorocarbon and helium forced convection design described in U.S. Pat. No. 5,131,233 achieves a volumetric power transfer density which is much greater than what is expected for workstation microprocessors even through the year 2010. However, due to the complexity of this system, the cost to implement this system is orders of magnitude above the $5 per microprocessor targeted by Intel and the PC manufacturers.
A typical characteristic of heat transfer devices for electronics today is that the atmosphere is the final heat sink of choice. Air cooling gives manufactures access to the broadest market of applications. Another typical characteristic of heat transfer devices for electronics today is that the semiconductor chip thermally contacts a passive aluminum spreader plate, which conducts the heat from the chip to one of several types of fins; these fins convect heat to the atmosphere with natural or forced convection.
As the power to be dissipated by semiconductor devices increases with time, a problem arises: within about ten years the thermal conductivity of the available materials becomes too low to conduct the heat from the semiconductor device to the fins with an acceptably low temperature drop. The thermal power density emerging from the chip will be so high in ten years than even copper or silver spreader plates will not be adequate. A clear and desirable solution to this problem is to develop inexpensive ways to manufacture more exotic materials like pyrolitic graphite or diamond that have even higher thermal conductivities. If the cost of these exotic materials does not fall quickly enough, an alternative solution is needed, such as will be discussed shortly.
Heat can be transported by an intermediate loop of recirculating fluid; heat from the hot object is conducted into a heat transfer fluid, the fluid is pumped by some means to a different location, and there the heat is conducted out of the fluid into a fin means and finally into the atmosphere. Thermosiphons use a change in density of the heat transfer fluid to impel circulation of the fluid, while heat pipes and boiling fluorocarbon use a phase transition of the heat transfer fluid to impel circulation of the fluid. While these approaches have important cooling applications, their cost for implementation will have to be reduced to generally impact semiconductor cooling. It is our suspicion that extracting the power for moving the heat transfer fluid from the heat flow itself is not energetically warranted in systems which dissipate hundreds of watts of waste heat from a semiconductor chip, and which dissipate several watts of electrical power by the fan circulating atmosphere through the fins.
Many heat transfer systems use an external source of energy to pump a recirculating heat transfer fluid. Most of these do not incorporate the pumped heat transfer fluid in an active spreader plate geometry that can be implemented as a replacement for a passive spreader plate. Most of these incur the cost disadvantage of requiring separate motors to impel the heat transfer fluid and to impel the atmosphere. Most of these incur the reliability disadvantage of using sealed shaft feed-throughs to deliver mechanical power to the heat transfer liquid. Most incur the added assembly cost and reliability exposure associated with hoses and fittings. None of these existing heat transfer systems simultaneously use a single motor to drive an impeller for the heat transfer fluid and an impeller for the atmosphere, use moving external magnetic fields to eliminate a rotary seal, and use monolithic assembly without hoses or fittings.
U.S. Pat. No. 1,819,528 describes a refrigerator. A motor sealed in the Freon reservoir compresses and circulates Freon through external plumbing coils, while simultaneously driving an atmosphere cooling fan using an eddy current based magnetic coupling. The internally generated moving magnetic fields cause the atmosphere moving fan to rotate more slowly that the Freon compressing impeller.
U.S. Pat. No. 4,519,447 describes an active spreader plate. The heat transfer fluid in the plate is impelled by magnetohydrodynamic pumping, which uses stationary magnetic fields plus large electric currents passing through the heat transfer fluid, as opposed to moving magnetic fields alone.
U.S. Pat. No. 5,183,104 describes a closed cycle expansion valve impingement cooling system for semiconductor chips. It utilizes hoses and fittings, and does not use moving external magnetic field to impel the heat transfer fluid.
U.S. Pat. No. 5,316,077 describes an active spreader plate. The heat transfer fluid in the active spreader plate is impelled by an impeller embedded in the active spreader plate that is driven by a sealed shaft passing through the plate.
U.S. Pat. No. 5,731,954 describes an electronics cooling system. The heat transfer fluid is impelled by an external pump, and the fluid circulates through discrete heat exchange elements through hoses and couplings.